a) Field of the Invention
The present invention is directed to a fixation mark that is presented to the patient to prevent unwanted eye movements during the treatment of an eye without taxing the patient's ability to concentrate.
b) Description of the Related Art
While the use of moving fixation marks in ophthalmologic devices is already known, this primarily relates to the determination of the visual field by means of perimetric arrangements and not to the suppression of unwanted eye movements while looking at a rigid fixation object during examination or treatment.
For example, DE 31 43 882 describes a method for ensuring fixation in ophthalmologic examination devices, particularly for determining the visual field. A fixation point is projected on a surface lying in the viewing direction of the patient and is coupled with a marking point to correlate its relative position with the visual field pattern to be established. The fixation point is periodically moved slowly on a predetermined path so that the patient can easily follow this movement. The path of the fixation point can be a straight line, an elongated ellipse, or a circular path. Excursion is advantageously +/−2° in horizontal direction and +/−0.2° in vertical direction with an excursion rate of about 2° per second. The instantaneous relative position of the fixation point with respect to the test mark can be transferred into the visual field model optically and electronically. Depending on the type of perimetric hemisphere that is used, the projection of the fixation point and test mark can be carried out through front projection or back projection.
A perimeter, particularly a hemispherical perimeter with a reflex fixation safeguard, is described in DE 41 08 403. In this case also, the visual field is analyzed by means of a fixation mark that is moved periodically relative to the patient. However, the movement is carried out by periodically rotating the hemispherical perimeter, together with the fixation mark which is rigidly connected to it, around an imaginary vertical axis substantially extending through the eye to be examined. The approximately circular light spot which can be deflected by two angular degrees can be projected on selected positions of the inner surface of the hemisphere by known electrically controllable systems. It is not described in detail how the visual field is determined from the results of the examination.
The solution described in U.S. Pat. No. 4,995,717 is also directed to a device for determining the visual field of a patient. For this purpose, a reference mark is placed in the center of a computer display on which the patient's eye must be fixated. Light marks are then generated successively on the computer display to determine the visual field of the patient. The patient signals, e.g., by actuating a mouse, when he/she can see the light mark as it becomes visible. The visual field and any defects of the eye being examined are determined by evaluating the detected light marks.
In contrast to the references mentioned thus far, DE 41 08 435 describes an arrangement for monitoring fixation which can preferably likewise be applied for devices for examination of the visual field, but principally also for other ophthalmologic instruments. Means are provided for rotating a structured fixation mark around a central axis to furnish a functional, reproducible fixation stimulus. The fixation mark has a structure and a predominant direction which can be identified by the test subject only when the test subject has foveal fixation. This is achieved in that the fixation mark is formed, for example, as a Landolt ring which adopts discrete directional orientations when rotated. A Geneva drive generates the discrete directional orientations of the fixation mark from a uniform rotating movement in 90-degree rotations with a stationary interval. The test subject must constantly follow the slit of the Landolt ring during the examination, which can be accomplished only with corresponding foveal fixation.
The solutions mentioned above are provided predominantly for campimetric examination, i.e., for determining the visual field or defects in the visual field. The fixation marks used in treatment devices are generally stationary. Although laser treatments in ophthalmology last only minutes, unwanted eye movements nevertheless occur when using stationary fixation marks. Also, this cannot be prevented by alternating the type, color and/or intensity of the fixation marks.